Textiles and clothing are part of everyday life. As such, realistic rendering of these articles has been an active area of computer graphics research for over a decade. In particular, modeling and rendering of knitwear poses a considerable challenge. Knitwear is constructed by spinning raw fibers into yarn. The yarn is then knitted into fabric based on a stitch pattern, and may optionally include a color pattern as well. The knitted fabric is then sewn into the desired clothing, such as a hat, scarf, or other type of clothing.
One obstacle to realistic knitwear generation is the complicated microstructure of yarn. Close examination of yarn reveals countless thin fibers which give knitwear a fuzzy appearance. In order to be accurate, a computer rendering of a close-up view of knitwear should show fuzzy yarn fibers. Improper rendering of delicate fuzzy yarn fiber structures can result in aliasing that causes a scene to look artificial. Rendering a down-like covering with existing techniques is inadequate and cumbersome in that its appearance changes in detail with different viewing distances.
Another difficulty presented by knitwear is its variations in shape. For knitwear lying flat, its microstructure has been efficiently displayed using prior art volume-rendering techniques; however, the only known techniques for rendering free-form knitwear with microstructure utilize Gouraud-shaded triangles, which was presented by H. Zhong, Y. Xu, B. Guo, and H. Shum, in the publication titled “Realistic and Efficient Rendering of Free-Form Knitwear”, published in the Journal of Visualization and Computer Animation, Special Issue on Cloth Simulation, in February 2001 (Zhong et al.). Techniques disclosed in Zhong et al., however, do not accurately handle close-up views.
Curved ray tracing, which is computationally expensive, does not effectively deal with multiple scattering that is especially evident on light-colored yarn. Curved ray tracing has a high rendering cost, and users must build different volumetric textures for different knitting patterns. This makes it difficult to model knitwear with advanced stitching patterns. Also, difficulties arise when the knitwear is severely deformed.
To avoid the high cost of ray tracing, volumetric objects can be rendered by hardware-aided texture mapping with transparency blending. Limitations arise, however, from the use of graphics hardware. Graphics hardware in general cannot compute the shading or shadowing for each individual texture pixel, with the exception of the GeForce 3 graphics hardware model that is provided through the NVIDIA Corporation having a place of business at 2701 San Tomas Expressway, Santa Clara, Calif. 95050, and which can do per-pixel lighting but cannot capture multiple scattering nor the attenuation effect of light passing through yarn. If a two dimensional (2D) alpha texture is employed, the volumetric texture can be split into slices parallel to the viewport and sorted from far to near for accurate blending computation. Splitting and sorting are solvable, but if shading and shadowing cannot be accurately computed according to the lighting and viewing conditions of the volumetric texture, the rendering result would likely appear artificial. A way to achieve realistic shading effects is to compute the results of all possible lighting and viewing conditions offline, and save them in a volumetric texture data structure. A moderately complex scene, however, requires a prohibitive amount of storage and computation resources.
Unlike woven fabrics, which can be well represented by specialized bidirectional reflectance (BRDF) models, knitwear is characterized by a macroscopic stitch structure that requires an explicit model of the knitting pattern and geometry. The microstructure of knitted yarn typically consists of a large number of thin fibers, where the size of yarn and stitches exceeds that of thread, thus precluding representation by BRDF models. Currently there are no convenient techniques for comprehensive photorealistic synthesis of knitwear.
Photorealistic rendering of a material that is arranged repetitively into a macrostructure poses another problem when fine-level interactions are apparent among the relatively smaller elements that make up the material. Examples of these smaller elements that make up a material include fibers in a yarn that have been knit into knitwear, or individual hairs that are on a human head. These relatively smaller elements exhibit reflection and transmission interactions, which include shadowing, occlusion, and multiple scattering, which need to be considered for physically realistic rendering. Such details should moreover be visible over the entire object and change in appearance for varying viewing distances. Additionally, rendering methods should accommodate arbitrary forms of the material. These difficulties together have not been addressed by previous rendering approaches so as to achieve photorealistic rendering of knitwear that efficiently handles free-form surfaces, varying levels of detail from different viewing distances, and complex stitch geometry.